The present invention relates to the field of telephones, and in particular to speakerphones. More particularly, the present invention relates to a control circuit for a telephone speakerphone wherein the loudspeaker and microphone signal channels have low gain states provided for compensating for ambient and channel noise levels.
In a typical speakerphone circuit, both an input transducer or microphone and an output transducer or loudspeaker are provided. It is not possible for a user to speak and listen to another party at the same time when using a speakerphone because both the microphone and loudspeaker cannot be enabled at the same time. This is because if both the microphone and loudspeaker are enabled at the same time, the speaker output signal would be fed back into the microphone, resulting in instability of the system due to feedback. Accordingly, circuitry is necessary to disable the microphone and enable the loudspeaker during the "listen" mode, and to enable the microphone and disable the loudspeaker during the "talk" mode. In telephone systems having an optional speakerphone in addition to the conventional handset, the use of the speakerphone is often termed "hands free" mode.
In speakerphone applications, it is necessary to be able to compensate for ambient noise levels in the environment in which the speakerphone is located and noise on the telephone line communication channel. Furthermore, it is necessary to be able to distinguish between actual voice signals generated by the microphone and "echo" signals present on the microphone channel due to the reflection of noise signals on the speaker channel onto the microphone channel by system deviations from ideal. The same is true of objectionable "sidetone" signals, which are reflected noise signals present on the speaker channel which result from noise present on the microphone channel. The need to compensate for noise is due to the obvious requirement that a speakerphone must be able to receive input signals over a wide area, as opposed to the conventional handset, wherein only signals spoken directly into the receiver are transmitted, and noise is not a problem, and wherein the ear piece is closely coupled to the human ear, so that again noise is not a problem. In contrast, in speakerphone applications, compensation for noise levels in the environment must be made if proper speakerphone operation is to be obtained.
Additionally, prior art speakerphones suffer from objectionable harsh clipping of the trailing or leading parts of voice signals when a transition is made between "talk" mode and "listen" mode, or vice versa.